[build] Set the default target platform version to the current beta versions. (#12525)

These are the versions we're going to target for .NET 6 (unless Apple manages to
release another Xcode before that).

Make.versions

@@ -80,8 +80,8 @@ MACCATALYST_NUGET_VERSION=15.0.100
 # Basically: this should be the last OS version of the platform in question when the current major .NET version is first released to stable.
 # Ref: https://github.com/dotnet/designs/blob/8e6394406d44f75f30ea2259a425cb9e38d75b69/accepted/2020/net5/net5.md#os-versions
 
-DEFAULT_TARGET_PLATFORM_VERSION_IOS=13.6
-DEFAULT_TARGET_PLATFORM_VERSION_TVOS=13.4
-DEFAULT_TARGET_PLATFORM_VERSION_WATCHOS=6.2
-DEFAULT_TARGET_PLATFORM_VERSION_MACOS=10.15
-DEFAULT_TARGET_PLATFORM_VERSION_MACCATALYST=13.5
+DEFAULT_TARGET_PLATFORM_VERSION_IOS=15.0
+DEFAULT_TARGET_PLATFORM_VERSION_TVOS=15.0
+DEFAULT_TARGET_PLATFORM_VERSION_WATCHOS=8.0
+DEFAULT_TARGET_PLATFORM_VERSION_MACOS=12.0
+DEFAULT_TARGET_PLATFORM_VERSION_MACCATALYST=15.0